This proposal focuses on co-rumination, a construct recently developed for this research program. Co-rumination refers to extensively discussing problems and is characterized by revisiting topics, speculating on causes and consequences of problems, and focusing on negative feelings. The friendship literature indicates that personal self-disclosure leads to close relationships; however, the coping literature indicates that focusing on problems leads to emotional difficulties. Co-rumination is a single construct that integrates both theoretical perspectives and is proposed to have seemingly contradictory outcomes. Co-rumination is expected to lead to close friendships but also to internalizing problems. In addition, girls are expected to co-ruminate more than boys, which is expected to contribute to girls having closer friendships than boys but also greater internalizing problems. These gender differences are expected to be stronger among adolescents than preadolescents. In an initial co- rumination study, a reliable survey measure of co-rumination in the friendships of children and adolescents was developed. Co-rumination was related to both friendship closeness and internalizing problems, and gender differences in co-rumination mediated gender differences in friendship closeness and internalizing problems, However, the study involved a single assessment and did not address the causal relation between co-rumination and adjustment, In the proposed research, approximately 1020 students in grades three, five, seven, and nine will participate in a short-term longitudinal study addressing whether co- rumination leads to increased friendship closeness and internalizing problems over time. The alternative hypothesis that friendship closeness and internalizing problems lead to increased co-rumination will also be tested. Gender and grade differences will also be examined. Participants will respond to surveys in their classrooms in the fall and spring of the school year. The long-terns objective of this work is to better understand processes by which close friendships and internalizing problems develop. This research could uncover a surprising pathway to internalizing problems, given the many benefits of close friendships. If this is true, incorporating the information in prevention and treatment programs could be crucial, especially for promoting well-being among girls.